The present invention relates to optical amplifiers using semiconductor optical amplifiers (SOAs) to amplify an optical signal. Such SOAs have wide applications in telecommunication networks as low-cost linear amplifiers for optical data signals. In particular the present invention relates to the use of a laser cavity to provide gain-clamping for increasing the linear region of operation and hence reducing interference noise.
Operation of a SOA outside the linear region of operation causes non-linear distortion and interference noise. In particular, at high output powers the gain reduces. Such gain modulation can cause non-linear distortion and interference noise in the time domain, that is inter-symbol interference, because the gain recovery time of a SOA is typically similar to the data modulation speed. Similarly, such gain modulation can cause interference noise in the frequency domain, that is inter-channel crosstalk between different frequency channels.
Gain-clamping using a laser cavity is a known technique to reduce gain modulation and the associated interference noise. Typically a laser cavity containing the active material of the SOA is provided to lase at a wavelength outside the desired signal band. In one known arrangement, wavelength dependant reflectors are arranged before and after the SOA to form a laser cavity longitudinally along the signal path. In another arrangement disclosed in U.S. Pat. No. 5,436,759, distributed Bragg reflectors are arranged to form a laser cavity extending vertically, that is perpendicular to the signal propagation axis and the layered structure of the SOA.
The lasing action in the laser cavity clamps the gain of the active material at the laser threshold. A clamped gain is therefore imposed on the amplification of signals in the signal band. By clamping the gain at a level below the normal unclamped gain, the linear region is extended to higher output powers. In particular, the linear region is extended to the saturation output power which, for a given bias current, has a higher level at the lower, clamped gain than at the higher, unclamped gain. This reduces the gain modulation and interference noise.
Gain clamping occurs at the expense of gain variability in that the gain is clamped at a value fixed at the point of manufacture. However it would be desirable provide a gain-clamped SOA for which the level of the clamped gain is controllable in use. There are many situations in telecommunications where this is desirable, for example to allow control the noise figure for the SOA which increases as the gain and hence the carrier density is reduced. Control of the clamped gain is also important in applications such as channel equalization and automatic power control. Automatic power control is becoming increasingly important within optical receivers to optimize the signal strength at the detector, particularly at data rates in excess of 10 GHz where automatic power control in the electronic domain can be prohibitively expensive.
The gain of a SOA which is not gain-clamped can be controlled by changing the bias current supplied to the SOA. However if the bias current is lowered to lower the gain, the saturation output power and hence the linear region also reduces. This contrasts with clamping the gain to a lower level using a laser cavity, in which case the saturation output power is increased at a given bias current, as explained above. What would be desirable is to provide for gain-clamping with variability of the gain at which clamping occurs so as to maximize the saturation output power.
According to the present invention, there is provided an optical amplifier comprising: a signal semiconductor optical amplifier having a waveguide forming at least part of a signal path between an input and an output, extending along a signal active region for amplification of a signal; a control active region of semiconductor material having a gain which is controllable independently from the gain of the signal active region; and a laser cavity containing both the signal active region and the control active region and being capable of clamping the gain of the signal active region, wherein the control active region is arranged not to amplify a signal in the signal path within a predetermined signal band.
In use, the signal active region amplifies the signal in a predetermined signal band passing along the signal path. The laser cavity fixes the total gain of both active regions because they are both contained in the laser cavity. The total gain is clamped at the laser threshold for the laser cavity, that is where the total gain is equal to the losses of the laser cavity, this being inherent in the lasing action.
In use, this arrangement allows the gain of the signal active region to be varied by controlling the gain of the control active region, for example by controlling the bias current supplied to the control active region. As the total gain within the lasing mode is limited to the loss within the cavity, changing the gain of the control active region causes an opposite change in the gain of the signal active region at the wavelength of the lasing mode. Thus a changed clamped gain is also imposed on the amplification of signals in the predetermined signal band which pass in the signal path through the signal active region.
Furthermore, as the control active region does not amplify a signal in the signal path within a predetermined signal band, it is possible to change the bias current to the control active region, or otherwise change the gain of the control active region, without changing the bias current supplied to the signal active region. Thus, the optical amplifier in accordance with the present invention allows signals to be amplified by the signal active region at a clamped gain which is variable by control of the control active region, so maximizing the saturation output power and hence the linear range.
Although the saturation output power of the control active region is changed, this has no effect on the saturation output power of the signal active region.
It is not necessary to change the bias current to the signal active region, although it is in principle possible to do so. Preferably, the signal active region is supplied with a maximum bias current in order to maximize the saturated output power for the signal channel.
Preferably, the laser cavity has a lasing mode at a wavelength outside the predetermined signal band. This is desirable to reduce the effect of the lasing mode on the signal passing through the signal active region, given that the lasing mode also propagates through the signal active region.
To control the wavelength of the lasing mode, the laser cavity may include a wavelength-dependent element, for example, a filter in the lasing cavity outside the signal path or a wavelength-selective coupler within the laser cavity, or wavelength-dependent reflectors to terminate the laser cavity.
The present may be embodied by several different types of optical amplifier.
In a first type of embodiment the control active region is the active region of a control SOA formed in a separate semiconductor chip from the signal SOA. This type of embodiment allows the optical amplifier to be constructed from optical components which, in themselves, are of known construction.
In a second type of embodiment, the signal active region and the control active region are different portions of the same semiconductor chip with the waveguide extending along both the signal active region and the control active region. This may be achieved, for example by at least one of the electrodes of the signal and control active regions being electrically isolated between the signal and the control active regions. One benefit of this arrangement is that it allows the entire optical amplifier to be integrated in a single semiconductor chip. Apart from the electrode configuration, the signal and control active region may in themselves have a conventional construction for an SOA. This allows the present invention to be easily applied to known SOA constructions. It results in an SOA having fundamentally the same properties as a known construction of SOA, but with a variable clamped gain.
In a third type of embodiment, the control active region is integrated outside the waveguide in the same semiconductor chip as the signal semiconductor optical amplifier. In a simple arrangement, the laser cavity extends transversely to the waveguide, preferably perpendicular to the layered structure of the semiconductor chip. This allows the signal and control active regions to be formed by respective layers of active material. In itself the laser cavity may have the construction of a known VCSEL (Vertical Cavity Surface Emitting Laser). This allows the provision of a relatively short laser cavity which provides several advantages including a quick response time. This type of embodiment also provides the advantage that the entire variable-gain-clamped optical amplifier may be integrated in a single semiconductor chip. Furthermore, the provision of the active regions as separate layers provide a high degree over of control over the characteristics and properties of the two active regions.
Two different techniques are used to prevent the control active region from amplifying a signal passing through the signal active region.
The first technique is for the control active region to be outside the signal path so that is does not amplify a signal in the signal path. This is intrinsic in the third type of embodiment. It may be implemented in the first type of embodiment by forming part of the laser cavity an optical path which is outside the signal path and in which the control active region is arranged. To couple that optical path into the signal path, in particular to the portion of the signal path containing the signal active region, one or more optical couplers may be provided in the signal path. The optical coupler(s) may be of any suitable form.
One preferred form is to use a wavelength-selective coupler such as a wavelength division multiplexing (WDM) coupler. This is preferred as the lowest loss implementation because the power of the signal path is not lost by being coupled into the laser cavity. The use of such a wavelength-selective coupler on the output side of the signal SOA also has the advantage that it prevents any transmission of the lasing mode onto the signal path towards the output of the optical amplifier.
As an alternative, the optical coupler(s) may be a wavelength-insensitive coupler such as a weighted beam splitter which splits (or combines) light at all wavelengths in a predetermined ratio. Such an arrangement is lossy, because some of the lasing mode is coupled into the signal path outside the laser cavity and some of the signal is coupled into the laser cavity, but may nonetheless be acceptable if there is sufficient gain in the lasing path. There is a cost-benefit in that wavelength-insensitive couplers are less expensive.
Various configurations for the laser cavity may be used. One simple configuration is a ring laser cavity in which the optical circuit provides a ring-shaped cavity around which the laser mode propagates. In this case, the laser cavity may include an isolator, or other directional element, to control the propagation direction of the lasing mode to be co-directional or counter-directional with respect to the signal path. Alternatively, the laser cavity may contain no directional elements to allow the lasing mode to propagate in both directions around the ring laser cavity. Control of the directionality of the laser cavity with respect to the signal path allows the optical power density to be equalized along the signal path and also gives control over the amount of amplified spontaneous emission (ASE) that is transmitted in the forward direction on the signal path.
To couple the ring laser into the signal path, the optical circuit may include a pair of optical couplers in the signal path on the input side and output side, respectively, of the signal semiconductor optical amplifier. In this case, the pair of optical couplers may couple respective ends of an optical path which contains the control semiconductor optical amplifier and which, together with the portion of the signal path between the pair of optical couplers, forms the ring laser cavity.
Another configuration for the laser cavity is a linear laser cavity in which the lasing mode is reflected at either end of the cavity to propagate back and forth. To couple the linear laser cavity into the signal path the optical circuit may include an optical coupler in the signal path. In this case, the optical coupler may couple an end of an optical path which contains the control semiconductor optical amplifier and which is terminated by a reflector to form one end of the linear laser cavity. The optical path containing the control SOA therefore forms part of the laser cavity, together with a portion of the signal path extending from the optical coupler and containing the signal SOA.
The other end of the linear laser cavity is preferably terminated by a reflector, such as a Bragg reflector, arranged in the signal path, although it is possible to terminate the laser cavity by a reflector arranged outside the signal path and coupled into the signal path by a second optical coupler.
The second technique to prevent the control active region from amplifying a signal passing along the signal path the control semiconductor optical amplifier has an insignificant gain in the predetermined signal band so that it does not amplify a signal in the predetermined signal band. This allows the control semiconductor optical amplifier to be arranged in the signal path which is beneficial because it allows for the optical amplifier to have a simpler construction. For example, the laser cavity may be terminated by reflectors arranged in the signal path outside the control and signal SOAs. The second technique is intrinsic in the second type of embodiment. However the second technique may be applied in combination with a first technique to particular advantage.
With the second technique, the control active region has a significant gain at the wavelength of the lasing mode, but an insignificant gain in the predetermined signal band. This may be achieved by selection of the gain profile of the control active region with respect to the gain profile of the signal active region which will be significant, and usually peak near the predetermined signal band. In particular, this may be achieved by providing the gain profile of the control active region to overlap with the gain profile of the signal active region at the wavelength of the lasing mode.
To allow better understanding, embodiments of the present invention will now be described by way of non-limitative example. With reference to the accompanying drawings, in which:
The various embodiments described below share a number of common elements which perform the same function in each embodiment. To avoid repetition, such common elements are referred to using the same reference numerals and a description thereof is not repeated.